Network Working Group P. Jones
Internet-Draft Cisco
Intended status: Standards Track D. Benham
Expires: March 8, 2019 C. Groves
Independent
September 4, 2018
A Solution Framework for Private Media in Privacy Enhanced RTP
Conferencing
draft-ietf-perc-private-media-framework-07
Abstract
This document describes a solution framework for ensuring that media
confidentiality and integrity are maintained end-to-end within the
context of a switched conferencing environment where media
distributors are not trusted with the end-to-end media encryption
keys. The solution aims to build upon existing security mechanisms
defined for the real-time transport protocol (RTP).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 8, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. PERC Entities and Trust Model . . . . . . . . . . . . . . . . 5
3.1. Untrusted Entities . . . . . . . . . . . . . . . . . . . 5
3.1.1. Media Distributor . . . . . . . . . . . . . . . . . . 6
3.1.2. Call Processing . . . . . . . . . . . . . . . . . . . 6
3.2. Trusted Entities . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Endpoint . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Key Distributor . . . . . . . . . . . . . . . . . . . 7
4. Framework for PERC . . . . . . . . . . . . . . . . . . . . . 7
4.1. End-to-End and Hop-by-Hop Authenticated Encryption . . . 8
4.2. E2E Key Confidentiality . . . . . . . . . . . . . . . . . 9
4.3. E2E Keys and Endpoint Operations . . . . . . . . . . . . 9
4.4. HBH Keys and Hop Operations . . . . . . . . . . . . . . . 10
4.5. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 10
4.5.1. Initial Key Exchange and Key Distributor . . . . . . 11
4.5.2. Key Exchange during a Conference . . . . . . . . . . 12
5. Authentication . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Identity Assertions . . . . . . . . . . . . . . . . . . . 13
5.2. Certificate Fingerprints in Session Signaling . . . . . . 13
5.3. Conferences Identification . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6.1. Third Party Attacks . . . . . . . . . . . . . . . . . . . 14
6.2. Media Distributor Attacks . . . . . . . . . . . . . . . . 15
6.2.1. Denial of service . . . . . . . . . . . . . . . . . . 15
6.2.2. Replay Attack . . . . . . . . . . . . . . . . . . . . 16
6.2.3. Delayed Playout Attack . . . . . . . . . . . . . . . 16
6.2.4. Splicing Attack . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 17
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. PERC Key Inventory . . . . . . . . . . . . . . . . . 19
A.1. DTLS-SRTP Exchange Yields HBH Keys . . . . . . . . . . . 20
A.2. The Key Distributor Transmits the KEK (EKT Key) . . . . . 20
A.3. Endpoints fabricate an SRTP Master Key . . . . . . . . . 21
A.4. Who has What Key . . . . . . . . . . . . . . . . . . . . 21
Appendix B. PERC Packet Format . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
Switched conferencing is an increasingly popular model for multimedia
conferences with multiple participants using a combination of audio,
video, text, and other media types. With this model, real-time media
flows from conference participants are not mixed, transcoded,
transrated, recomposed, or otherwise manipulated by a Media
Distributor, as might be the case with a traditional media server or
multipoint control unit (MCU). Instead, media flows transmitted by
conference participants are simply forwarded by the Media Distributor
to each of the other participants, often forwarding only a subset of
flows based on voice activity detection or other criteria. In some
instances, the Media Distributors may make limited modifications to
RTP [RFC3550] headers, for example, but the actual media content
(e.g., voice or video data) is unaltered.
An advantage of switched conferencing is that Media Distributors can
be more easily deployed on general-purpose computing hardware,
including virtualized environments in private and public clouds.
Deploying conference resources in a public cloud environment might
introduce a higher security risk. Whereas traditional conference
resources were usually deployed in private networks that were
protected, cloud-based conference resources might be viewed as less
secure since they are not always physically controlled by those who
use them. Additionally, there are usually several ports open to the
public in cloud deployments, such as for remote administration, and
so on.
This document defines a solution framework wherein media privacy is
ensured by making it impossible for a media distributor to gain
access to keys needed to decrypt or authenticate the actual media
content sent between conference participants. At the same time, the
framework allows for the Media Distributors to modify certain RTP
headers; add, remove, encrypt, or decrypt RTP header extensions; and
encrypt and decrypt RTCP packets. The framework also prevents replay
attacks by authenticating each packet transmitted between a given
participant and the media distributor using a unique key per endpoint
that is independent from the key for media encryption and
authentication.
A goal of this document is to define a framework for enhanced privacy
in RTP-based conferencing environments while utilizing existing
security procedures defined for RTP with minimal enhancements.
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2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] when they
appear in ALL CAPS. These words may also appear in this document in
lower case as plain English words, absent their normative meanings.
Additionally, this solution framework uses the following terms and
acronyms:
End-to-End (E2E): Communications from one endpoint through one or
more Media Distributors to the endpoint at the other end.
Hop-by-Hop (HBH): Communications between an endpoint and a Media
Distributor or between Media Distributors.
Trusted Endpoint: An RTP flow terminating entity that has possession
of E2E media encryption keys and terminates E2E encryption. This may
include embedded user conferencing equipment or browsers on
computers, media gateways, MCUs, media recording device and more that
are in the trusted domain for a given deployment.
Media Distributor (MD): An RTP middlebox that is not allowed to to
have access to E2E encryption keys. It operates according to the
Selective Forwarding Middlebox RTP topologies [RFC7667] per the
constraints defined by the PERC system, which includes, but not
limited to, having no access to RTP media unencrypted and having
limits on what RTP header field it can alter.
Key Distributor: An entity that is a logical function which
distributes keying material and related information to trusted
endpoints and Media Distributor(s), only that which is appropriate
for each. The Key Distributor might be co-resident with another
entity trusted with E2E keying material.
Conference: Two or more participants communicating via trusted
endpoints to exchange RTP flows through one or more Media
Distributor.
Call Processing: All trusted endpoints in the conference connect to
it by a call processing dialog, such as with the Focus defined in the
Framework for Conferencing with SIP [RFC4353].
Third Party: Any entity that is not an Endpoint, Media Distributor,
Key Distributor or Call Processing entity as described in this
document.
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3. PERC Entities and Trust Model
The following figure depicts the trust relationships, direct or
indirect, between entities described in the subsequent sub-sections.
Note that these entities may be co-located or further divided into
multiple, separate physical devices.
Please note that some entities classified as untrusted in the simple,
general deployment scenario used most commonly in this document might
be considered trusted in other deployments. This document does not
preclude such scenarios, but will keep the definitions and examples
focused by only using the the simple, most general deployment
scenario.
|
+----------+ | +-----------------+
| Endpoint | | | Call Processing |
+----------+ | +-----------------+
|
|
+----------------+ | +--------------------+
| Key Distributor| | | Media Distributor |
+----------------+ | +--------------------+
|
Trusted | Untrusted
Entities | Entities
|
Figure 1: Trusted and Untrusted Entities in PERC
3.1. Untrusted Entities
The architecture described in this framework document enables
conferencing infrastructure to be hosted in domains, such as in a
cloud conferencing provider's facilities, where the trustworthiness
is below the level needed to assume the privacy of participant's
media will not be compromised. The conferencing infrastructure in
such a domain is still trusted with reliably connecting the
participants together in a conference, but not trusted with keying
material needed to decrypt any of the participant's media. Entities
in such lower trustworthiness domains will simply be referred to as
untrusted entities from this point forward.
It is important to understand that untrusted in this document does
not mean an entity is not expected to function properly. Rather, it
means only that the entity does not have access to the E2E media
encryption keys.
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3.1.1. Media Distributor
A Media Distributor forwards RTP flows between endpoints in the
conference while performing per-hop authentication of each RTP
packet. The Media Distributor may need access to one or more RTP
headers or header extensions, potentially adding or modifying a
certain subset. The Media Distributor will also relay secured
messaging between the endpoints and the Key Distributor and will
acquire per-hop key information from the Key Distributor. The actual
media content MUST NOT not be decryptable by a Media Distributor, so
it is untrusted to have access to the E2E media encryption keys. The
key exchange mechanisms specified in this framework will prevent the
Media Distributor from gaining access to the E2E media encryption
keys.
An endpoint's ability to join a conference hosted by a Media
Distributor MUST NOT alone be interpreted as being authorized to have
access to the E2E media encryption keys, as the Media Distributor
does not have the ability to determine whether an endpoint is
authorized. Instead, the Key Distributor is responsible for
authenticating endpoints (e.g., using WebRTC Identity
[I-D.ietf-rtcweb-security-arch]) and determining their authorization
to receive E2E media encryption keys.
A Media Distributor MUST perform its role in properly forwarding
media packets while taking measures to mitigate the adverse effects
of denial of service attacks (refer to Section 6), etc, to a level
equal to or better than traditional conferencing (i.e. non-PERC)
deployments.
A Media Distributor or associated conferencing infrastructure may
also initiate or terminate various conference control related
messaging, which is outside the scope of this framework document.
3.1.2. Call Processing
The call processing function is untrusted in the simple, general
deployment scenario. When a physical subset of the call processing
function resides in facilities outside the trusted domain, it should
not be trusted to have access to E2E key information.
The call processing function may include the processing of call
signaling messages, as well as the signing of those messages. It may
also authenticate the endpoints for the purpose of call signaling and
subsequently joining of a conference hosted through one or more Media
Distributors. Call processing may optionally ensure the privacy of
call signaling messages between itself, the endpoint, and other
entities.
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In any deployment scenario where the call processing function is
considered trusted, the call processing function MUST ensure the
integrity of received messages before forwarding to other entities.
3.2. Trusted Entities
From the PERC model system perspective, entities considered trusted
(refer to Figure 1) can be in possession of the E2E media encryption
keys for one or more conferences.
3.2.1. Endpoint
An endpoint is considered trusted and will have access to E2E key
information. While it is possible for an endpoint to be compromised,
subsequently performing in undesired ways, defining endpoint
resistance to compromise is outside the scope of this document.
Endpoints will take measures to mitigate the adverse effects of
denial of service attacks (refer to Section 6) from other entities,
including from other endpoints, to a level equal to or better than
traditional conference (i.e., non-PERC) deployments.
3.2.2. Key Distributor
The Key Distributor, which may be colocated with an endpoint or exist
standalone, is responsible for providing key information to endpoints
for both end-to-end and hop-by-hop security and for providing key
information to Media Distributors for the hop-by-hop security.
Interaction between the Key Distributor and the call processing
function is necessary to for proper conference-to-endpoint mappings.
This is described in Section 5.3.
The Key Distributor needs to be secured and managed in a way to
prevent exploitation by an adversary, as any kind of compromise of
the Key Distributor puts the security of the conference at risk.
4. Framework for PERC
The purpose for this framework is to define a means through which
media privacy can be ensured when communicating within a conferencing
environment consisting of one or more Media Distributors that only
switch, hence not terminate, media. It does not otherwise attempt to
hide the fact that a conference between endpoints is taking place.
This framework reuses several specified RTP security technologies,
including SRTP [RFC3711], PERC EKT [I-D.ietf-perc-srtp-ekt-diet], and
DTLS-SRTP [RFC5764].
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4.1. End-to-End and Hop-by-Hop Authenticated Encryption
This solution framework focuses on the end-to-end privacy and
integrity of the participant's media by limiting access of the end-
to-end key information to trusted entities. However, this framework
does give a Media Distributor access to RTP headers and all or most
header extensions, as well as the ability to modify a certain subset
of those headers and to add header extensions. Packets received by a
Media Distributor or an endpoint are authenticated hop-by-hop.
To enable all of the above, this framework defines the use of two
security contexts and two associated encryption keys: an "inner" key
(an E2E key distinct for each transmitted media flow) for
authenticated encryption of RTP media between endpoints and an
"outer" key (HBH key) known only to media distributor and the
adjacent endpoint) for the hop between an endpoint and a Media
Distributor or between Media Distributor.
+-------------+ +-------------+
| |################################| |
| Media |------------------------ *----->| Media |
| Distributor |
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for a description of the keys used in PERC and Appendix B for an
overview of how the packet appears on the wire.
RTCP can only be encrypted hop-by-hop, not end-to-end. This
framework introduces no additional step for RTCP authenticated
encryption, so the procedures needed are specified in [RFC3711] and
use the same outer, hop-by-hop cryptographic context chosen in the
Double operation described above.
4.2. E2E Key Confidentiality
To ensure the confidentiality of E2E keys shared between endpoints,
endpoints will make use of a common Key Encryption Key (KEK) that is
known only by the trusted entities in a conference. That KEK,
defined in the PERC EKT [I-D.ietf-perc-srtp-ekt-diet] as the EKT Key,
will be used to subsequently encrypt the SRTP master key used for E2E
authenticated encryption of media sent by a given endpoint. Each
endpoint in the conference will create a random SRTP master key for
E2E authenticated encryption, thus participants in the conference
MUST keep track of the E2E keys received via the Full EKT Field for
each distinct SSRC in the conference so that it can properly decrypt
received media. Note, too, that an endpoint may change its E2E key
at any time and advertise that new key to the conference as specified
in [I-D.ietf-perc-srtp-ekt-diet].
4.3. E2E Keys and Endpoint Operations
Any given RTP media flow can be identified by its SSRC, and endpoints
might send more than one at a time and change the mix of media flows
transmitted during the life of a conference.
Thus, endpoints MUST maintain a list of SSRCs from received RTP flows
and each SSRC's associated E2E key information. Following a change
in an E2E key, prior E2E keys SHOULD be retained by receivers for a
period long enough to ensure that late-arriving or out-of-order
packets from the endpoint can be successfully decrypted. Receiving
endpoints MUST discard old E2E keys no later than when it leaves the
conference.
If there is a need to encrypt one or more RTP header extensions end-
to-end, an encryption key is derived from the end-to-end SRTP master
key to encrypt header extensions as per [RFC6904]. The Media
Distributor will not be able use the information contained in those
header extensions encrypted with an E2E key.
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4.4. HBH Keys and Hop Operations
To ensure the integrity of transmitted media packets, this framework
requires that every packet be authenticated hop-by-hop (HBH) between
an endpoint and a Media Distributor, as well between Media
Distributors. The authentication key used for hop-by-hop
authentication is derived from an SRTP master key shared only on the
respective hop. Each HBH key is distinct per hop and no two hops
ever use the same SRTP master key.
Using hop-by-hop authentication gives the Media Distributor the
ability to change certain RTP header values. Which values the Media
Distributor can change in the RTP header are defined in
[I-D.ietf-perc-double]. RTCP can only be encrypted HBH, giving the
Media Distributor the flexibility to forward RTCP content unchanged,
transmit compound RTCP packets or to initiate RTCP packets for
reporting statistics or conveying other information. Performing hop-
by-hop authentication for all RTP and RTCP packets also helps provide
replay protection (see Section 6).
If there is a need to encrypt one or more RTP header extensions hop-
by-hop, an encryption key is derived from the hop-by-hop SRTP master
key to encrypt header extensions as per [RFC6904]. This will still
give the Media Distributor visibility into header extensions, such as
the one used to determine audio level [RFC6464] of conference
participants. Note that when RTP header extensions are encrypted,
all hops - in the untrusted domain at least - will need to decrypt
and re-encrypt these encrypted header extensions.
4.5. Key Exchange
In brief, the keys used by any given endpoints are determined in the
following way:
o The HBH keys that the endpoint uses to send and receive SRTP media
are derived from a DTLS handshake that the endpoint performs with
the Key Distributor (following normal DTLS-SRTP procedures).
o The E2E key that an endpoint uses to send SRTP media can either be
set from DTLS or chosen by the endpoint. It is then distributed
to other endpoints in a Full EKT Field, encrypted under an EKTKey
provided to the client by the Key Distributor within the DTLS
channel they negotiated.
o Each E2E key that an endpoint uses to receive SRTP media is set by
receiving a Full EKT Field from another endpoint.
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4.5.1. Initial Key Exchange and Key Distributor
The Media Distributor maintains a tunnel with the Key Distrubutor
(e.g., using [I-D.ietf-perc-dtls-tunnel]), making it possible for the
Media Distributor to facilitate the establishment of a secure DTLS
association between each endpoint and the Key Distributor as shown
the following figure. The DTLS association between endpoints and the
Key Distributor will enable each endpoint to generate E2E and HBH
keys and receive the Key Encryption Key (KEK) (i.e., EKT Key). At
the same time, the Key Distributor can securely provide the HBH key
information to the Media Distributor. The key information summarized
here may include the SRTP master key, SRTP master salt, and the
negotiated cryptographic transform.
+-----------+
KEK info | Key | HBH Key info to
to Endpoints |Distributor| Endpoints & Media Distributor
+-----------+
# ^ ^ #
# | | #--- Tunnel
# | | #
+-----------+ +-----------+ +-----------+
| Endpoint | DTLS | Media | DTLS | Endpoint |
| KEK || KEK |
| HBH Key | to Key Dist | HBH Keys | to Key Dist | HBH Key |
+-----------+ +-----------+ +-----------+
Figure 3: Exchanging Key Information Between Entities
Endpoints will establish a DTLS-SRTP [RFC5764] association over the
RTP session's media ports for the purposes of key information
exchange with the Key Distributor. The Media Distributor will not
terminate the DTLS signaling, but will instead forward DTLS packets
received from an endpoint on to the Key Distributor (and vice versa)
via a tunnel established between Media Distributor and the Key
Distributor. This tunnel is used to encapsulate the DTLS-SRTP
signaling between the Key Distributor and endpoints will also be used
to convey HBH key information from the Key Distributor to the Media
Distributor, so no additional protocol or interface is required.
In establishing the DTLS association between endpoints and the Key
Distributor, the endpoint MUST act as the DTLS client and the Key
Distributor MUST act as the DTLS server. The Key Encryption Key
(KEK) (i.e., EKT Key) is conveyed by the Key Distributor over the
DTLS association to endpoints via procedures defined in PERC EKT
[I-D.ietf-perc-srtp-ekt-diet] via the EKTKey message.
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Note that following DTLS-SRTP procedures for the
[I-D.ietf-perc-double] cipher, the endpoint will generate both E2E
and HBH encryption keys and salt values. Endpoints MAY use the DTLS-
SRTP generated E2E key for transmission or MAY generate a fresh E2E
key. In either case, the generated SRTP master salt for E2E
encryption MUST be replaced with the salt value provided by the Key
Distributor via the EKTKey message. That is because every endpoint
in the conference uses the same SRTP master salt. The endpoint only
transmits the SRTP master key (not the salt) used for E2E encryption
to other endpoints in RTP/RTCP packets per
[I-D.ietf-perc-srtp-ekt-diet].
Media Distributors use DTLS-SRTP [RFC5764] directly with a peer Media
Distributor to establish the HBH key for transmitting RTP and RTCP
packets to that peer Media Distributor. The Key Distributor does not
facilitate establishing a HBH key for use between Media Distributors.
4.5.2. Key Exchange during a Conference
Following the initial key information exchange with the Key
Distributor, an endpoint will be able to encrypt media end-to-end
with an E2E key, sending that E2E key to other endpoints encrypted
with the KEK, and will be able to encrypt and authenticate RTP
packets using a HBH key. The procedures defined do not allow the
Media Distributor to gain access to the KEK information, preventing
it from gaining access to any endpoint's E2E key and subsequently
decrypting media.
The KEK (i.e., EKT Key) may need to change from time-to-time during
the life of a conference, such as when a new participant joins or
leaves a conference. Dictating if, when or how often a conference is
to be re-keyed is outside the scope of this document, but this
framework does accommodate re-keying during the life of a conference.
When a Key Distributor decides to re-key a conference, it transmits a
specific message defined in PERC EKT [I-D.ietf-perc-srtp-ekt-diet] to
each of the conference participants. The endpoint MUST create a new
SRTP master key and prepare to send that key inside a Full EKT Field
using the new EKTKey. Since it may take some time for all of the
endpoints in conference to finish re-keying, senders MUST delay a
short period of time before sending media encrypted with the new
master key, but it MUST be prepared to make use of the information
from a new inbound EKT Key immediately. See Section 2.2.2 of
[I-D.ietf-perc-srtp-ekt-diet].
Endpoints MAY follow the procedures in section 5.2 of [RFC5764] to
re-negotiate HBH keys as desired. If new HBH keys are generated, the
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new keys are also delivered to the Media Distributor following the
procedures defined in [I-D.ietf-perc-dtls-tunnel].
Endpoints are at liberty to change the E2E encryption key used at any
time. Endpoints MUST generate a new E2E encryption key whenever it
receives a new EKT Key. After switching to a new key, the new key
will be conveyed to other endpoints in the conference in RTP/RTCP
packets per [I-D.ietf-perc-srtp-ekt-diet].
5. Authentication
It is important to this solution framework that the entities can
validate the authenticity of other entities, especially the Key
Distributor and endpoints. The details of this are outside the scope
of specification but a few possibilities are discussed in the
following sections. The key requirements is that endpoints can
verify they are connected to the correct Key Distributor for the
conference and the Key Distributor can verify the endpoints are the
correct endpoints for the conference.
Two possible approaches to solve this are Identity Assertions and
Certificate Fingerprints.
5.1. Identity Assertions
WebRTC Identity assertion [I-D.ietf-rtcweb-security-arch] can be used
to bind the identity of the user of the endpoint to the fingerprint
of the DTLS-SRTP certificate used for the call. This certificate is
unique for a given call and a conference. This allows the Key
Distributor to ensure that only authorized users participate in the
conference. Similarly the Key Distributor can create a WebRTC
Identity assertion to bind the fingerprint of the unique certificate
used by the Key Distributor for this conference so that the endpoint
can validate it is talking to the correct Key Distributor. Such a
setup requires an Identity Provider (Idp) trusted by the endpoints
and the Key Distributor.
5.2. Certificate Fingerprints in Session Signaling
Entities managing session signaling are generally assumed to be
untrusted in the PERC framework. However, there are some deployment
scenarios where parts of the session signaling may be assumed
trustworthy for the purposes of exchanging, in a manner that can be
authenticated, the fingerprint of an entity's certificate.
As a concrete example, SIP [RFC3261] and SDP [RFC4566] can be used to
convey the fingerprint information per [RFC5763]. An endpoint's SIP
User Agent would send an INVITE message containing SDP for the media
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session along with the endpoint's certificate fingerprint, which can
be signed using the procedures described in [RFC4474] for the benefit
of forwarding the message to other entities by the Focus [RFC4353].
Other entities can now verify the fingerprints match the certificates
found in the DTLS-SRTP connections to find the identity of the far
end of the DTLS-SRTP connection and check that is the authorized
entity.
Ultimately, if using session signaling, an endpoint's certificate
fingerprint would need to be securely mapped to a user and conveyed
to the Key Distributor so that it can check that that user is
authorized. Similarly, the Key Distributor's certificate fingerprint
can be conveyed to endpoint in a manner that can be authenticated as
being an authorized Key Distributor for this conference.
5.3. Conferences Identification
The Key Distributor needs to know what endpoints are being added to a
given conference. Thus, the Key Distributor and the Media
Distributor will need to know endpoint-to-conference mappings, which
is enabled by exchanging a conference-specific unique identifier as
defined in [I-D.ietf-perc-dtls-tunnel]. How this unique identifier
is assigned is outside the scope of this document.
6. Security Considerations
This framework, and the individual protocols defined to support it,
must take care to not increase the exposure to Denial of Service
(DoS) attacks by untrusted or third-party entities and should take
measures to mitigate, where possible, more serious DoS attacks from
on-path and off-path attackers.
The following section enumerates the kind of attacks that will be
considered in the development of this framework's solution.
6.1. Third Party Attacks
On-path attacks are mitigated by HBH integrity protection and
encryption. The integrity protection mitigates packet modification
and encryption makes selective blocking of packets harder, but not
impossible.
Off-path attackers may try connecting to different PERC entities and
send specifically crafted packets. A successful attacker might be
able to get the Media Distributor to forward such packets. If not
making use of HBH authentication on the Media Distributor, such an
attack could only be detected in the receiving endpoints where the
forged packets would finally be dropped.
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Another potential attack is a third party claiming to be a Media
Distributor, fooling endpoints in to sending packets to the false
Media Distributor instead of the correct one. The deceived sending
endpoints could incorrectly assuming their packets have been
delivered to endpoints when they in fact have not. Further, the
false Media Distributor may cascade to another legitimate Media
Distributor creating a false version of the real conference.
This attack can be mitigated by the false Media Distributor not being
authenticated by the Key Distributor during PERC Tunnel
establishment. Without the tunnel in place, endpoints will not
establish secure associations with the Key Distributor and receive
the KEK, causing the conference to not proceed.
6.2. Media Distributor Attacks
The Media Distributor can attack the session in a number of possible
ways.
6.2.1. Denial of service
Any modification of the end-to-end authenticated data will result in
the receiving endpoint getting an integrity failure when performing
authentication on the received packet.
The Media Distributor can also attempt to perform resource
consumption attacks on the receiving endpoint. One such attack would
be to insert random SSRC/CSRC values in any RTP packet with an inband
key-distribution message attached (i.e., Full EKT Field). Since such
a message would trigger the receiver to form a new cryptographic
context, the Media Distributor can attempt to consume the receiving
endpoints resources.
Another denial of service attack is where the Media Distributor
rewrites the PT field to indicate a different codec. The effect of
this attack is that any payload packetized and encoded according to
one RTP payload format is then processed using another payload format
and codec. Assuming that the implementation is robust to random
input, it is unlikely to cause crashes in the receiving software/
hardware. However, it is not unlikely that such rewriting will cause
severe media degradation.
For audio formats, this attack is likely to cause highly disturbing
audio and/or can be damaging to hearing and playout equipment.
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6.2.2. Replay Attack
Replay attack is when an already received packets from a previous
point in the RTP stream is replayed as new packet. This could, for
example, allow a Media Distributor to transmit a sequence of packets
identified as a user saying "yes", instead of the "no" the user
actually said.
The mitigation for a replay attack is to prevent old packets beyond a
small-to-modest jitter and network re-ordering sized window to be
rejected. End-to-end replay protection MUST be provided for the
whole duration of the conference.
6.2.3. Delayed Playout Attack
The delayed playout attack is a variant of the replay attack. This
attack is possible even if E2E replay protection is in place.
However, due to fact that the Media Distributor is allowed to select
a sub-set of streams and not forward the rest to a receiver, such as
in forwarding only the most active speakers, the receiver has to
accept gaps in the E2E packet sequence. The issue with this is that
a Media Distributor can select to not deliver a particular stream for
a while.
Within the window from last packet forwarded to the receiver and the
latest received by the Media Distributor, the Media Distributor can
select an arbitrary starting point when resuming forwarding packets.
Thus what the media source said can be substantially delayed at the
receiver with the receiver believing that it is what was said just
now, and only delayed due to transport delay.
6.2.4. Splicing Attack
The splicing attack is an attack where a Media Distributor receiving
multiple media sources splices one media stream into the other. If
the Media Distributor is able to change the SSRC without the receiver
having any method for verifying the original source ID, then the
Media Distributor could first deliver stream A and then later forward
stream B under the same SSRC as stream A was previously using. Not
allowing the Media Distributor to change the SSRC mitigates this
attack.
7. IANA Considerations
There are no IANA considerations for this document.
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8. Acknowledgments
The authors would like to thank Mo Zanaty and Christian Oien for
invaluable input on this document. Also, we would like to
acknowledge Nermeen Ismail for serving on the initial versions of
this document as a co-author.
9. References
9.1. Normative References
[I-D.ietf-perc-double]
Jennings, C., Jones, P., Barnes, R., and A. Roach, "SRTP
Double Encryption Procedures", draft-ietf-perc-double-09
(work in progress), May 2018.
[I-D.ietf-perc-dtls-tunnel]
Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
between a Media Distributor and Key Distributor to
Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-03
(work in progress), April 2018.
[I-D.ietf-perc-srtp-ekt-diet]
Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
Andreasen, "Encrypted Key Transport for DTLS and Secure
RTP", draft-ietf-perc-srtp-ekt-diet-08 (work in progress),
July 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, .
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904,
DOI 10.17487/RFC6904, April 2013,
.
9.2. Informative References
[I-D.ietf-rtcweb-security-arch]
Rescorla, E., "WebRTC Security Architecture", draft-ietf-
rtcweb-security-arch-15 (work in progress), July 2018.
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[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
DOI 10.17487/RFC4353, February 2006,
.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, .
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
2010, .
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764,
DOI 10.17487/RFC5764, May 2010,
.
[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464,
DOI 10.17487/RFC6464, December 2011,
.
[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
.
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Appendix A. PERC Key Inventory
PERC specifies the use of a number of different keys and,
understandably, it looks complicated or confusing on the surface.
This section summarizes the various keys used in the system, how they
are generated, and what purpose they serve.
The keys are described in the order in which they would typically be
acquired.
The various keys used in PERC are shown in Figure 4 below.
+-----------+----------------------------------------------------+
| Key | Description |
+-----------+----------------------------------------------------+
| KEK | Key shared by all endpoints and used to encrypt |
| (EKT Key) | each endpoint's SRTP master key so receiving |
| | endpoints can decrypt media. |
+-----------+----------------------------------------------------+
| HBH Key | Key used to encrypt media hop-by-hop. |
+-----------+----------------------------------------------------+
| E2E Key | Key used to encrypt media end-to-end. |
+-----------+----------------------------------------------------+
Figure 4: Key Inventory
As you can see, the number key types is very small. However, what
can be challenging is keeping track of all of the distinct E2E keys
as the conference grows in size. With 1,000 participants in a
conference, there will be 1,000 distinct SRTP master keys, all of
which share the same master salt. Each of those keys are passed
through the KDF defined in [RFC3711] to produce the actual encryption
and authentication keys. Complicating key management is the fact
that the KEK can change and, when it does, the endpoints generate new
SRTP master keys. And, of course, there is a new SRTP master salt to
go with those keys. Endpoints have to retain old keys for a period
of time to ensure they can properly decrypt late-arriving or out-of-
order packets.
The time required to retain old keys (either EKT Keys or SRTP master
keys) is not specified, but they should be retained at least for the
period of time required to re-key the conference or handle late-
arriving or out-of-order packets. A period of 60s should be
considered a generous retention period, but endpoints may keep old
keys on hand until the end of the conference.
Or more detailed explanation of each of the keys follows.
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A.1. DTLS-SRTP Exchange Yields HBH Keys
The first set of keys acquired are for hop-by-hop encryption and
decryption. Assuming the use of Double [I-D.ietf-perc-double], the
endpoint would perform DTLS-SRTP exchange with the key distributor
and receive a key that is, in fact, "double" the size that is needed.
Per the Double specification, the E2E part is the first half of the
key, so the endpoint will just discard that information in PERC. It
is not used. The second half of the key material is for HBH
operations, so that half of the key (corresponding to the least
significant bits) is assigned internally as the HBH key.
The media distributor doesn't perform DTLS-SRTP, but it is at this
point that the key distributor will inform the media distributor of
the HBH key value via the tunnel protocol
([I-D.ietf-perc-dtls-tunnel]). The key distributor will send the
least significant bits corresponding to the half of the keying
material determined through DTLS-SRTP with the endpoint to the media
distributor via the tunnel protocol. There is a salt generated along
with the HBH key. The salt is also longer than needed for HBH
operations, thus only the least significant bits of the required
length (i.e., half of the generated salt material) are sent to the
media distributor via the tunnel protocol.
No two endpoints will have the same HBH key, thus the media
distributor must keep track each distinct HBH key (and the
corresponding salt) and use it only for the specified hop.
This key is also used for HBH encryption of RTCP. RTCP is not end-
to-end encrypted in PERC.
A.2. The Key Distributor Transmits the KEK (EKT Key)
Via the aforementioned DTLS-SRTP association, the key distributor
will send the endpoint the KEK (i.e., EKT Key per
[I-D.ietf-perc-srtp-ekt-diet]). This key is known only to the key
distributor and endpoints. This key is the most important to protect
since having knowledge of this key (and the SRTP master salt
transmitted as a part of the same message) will allow an entity to
decrypt any media packet in the conference.
Note that the key distributor can send any number of EKT Keys to
endpoints. This can be used to re-key the entire conference. Each
key is identified by a "Security Parameter Index" (SPI) value.
Endpoints should expect that a conference might be re-keyed when a
new participant joins a conference or when a participant leaves a
conference in order to protect the confidentiality of the
conversation before and after such events.
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The SRTP master salt to be used by the endpoint is transmitted along
with the EKT Key. All endpoints in the conference utilize the same
SRTP master salt that corresponds with a given EKT Key.
The EKT Field in media packets is encrypted using a cipher specified
via the EKTKey message (e.g., AES Key Wrap with a 128-bit key). This
cipher is different than the cipher used to protect media and is only
used to encrypt the endpoint's SRTP master key (and other EKT Field
data as per [I-D.ietf-perc-srtp-ekt-diet]).
The media distributor is not given the KEK (i.e., EKT Key).
A.3. Endpoints fabricate an SRTP Master Key
As stated earlier, the E2E key determined via DTLS-SRTP MAY be
discarded in favor of a locally-generated SRTP master key. While the
DTLS-SRTP-derived key could be used, the fact that an endpoint might
need to change the SRTP master key periodically or is forced to
change the SRTP master key as a result of the EKT key changing means
using it has only limited utility. To reduce complexity, PERC
*RECOMMENDS* that endpoints create random SRTP master keys locally to
be used for E2E encryption.
This locally-generated SRTP master key is used along with the master
salt transmitted to the endpoint from the key distributor via the
EKTKey message to encrypt media end-to-end.
Since the media distributor is not involved in E2E functions, it will
not create this key nor have access to any endpoint's E2E key. Note,
too, that even the key distributor is unaware of the locally-
generated E2E keys used by each endpoint.
The endpoint will transmit its E2E key to other endpoints in the
conference by periodically including it in SRTP packets in a Full EKT
Field. When placed in the Full EKT Field, it is encrypted using the
EKT Key provided by the key distributor. The master salt is not
transmitted, though, since all endpoints will have received the same
master salt via the EKTKey message. The recommended frequency with
which an endpoint transmits its SRTP master key is specified in
[I-D.ietf-perc-srtp-ekt-diet].
A.4. Who has What Key
All endpoints have knowledge of the KEK.
Every HBH key is distinct for a given endpoint, thus Endpoint A and
endpoint B do not have knowledge of the other's HBH key.
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Each endpoint generates its own E2E Key (SRTP master key), thus the
key distinct per endpoint. This key is transmitted (encrypted) via
the EKT Field to other endpoints. Endpoints that receive media from
a given transmitting endpoint will therefore have knowledge of the
transmitter's E2E key.
To summarize the various keys and which entity is in possession of a
given key, refer to Figure 5.
+----------------------+------------+-------+-------+------------+
| Key / Entity | Endpoint A | MD X | MD Y | Endpoint B |
+----------------------+------------+-------+-------+------------+
| KEK | Yes | No | No | Yes |
+----------------------+------------+-------+-------+------------+
| E2E Key (A and B) | Yes | No | No | Yes |
+----------------------+------------+-------+-------+------------+
| HBH Key (A<=>MD X) | Yes | Yes | No | No |
+----------------------+------------+-------+-------+------------+
| HBH Key (B<=>MD Y) | No | No | Yes | Yes |
+----------------------+------------+---------------+------------+
| HBH Key (MD X<=>MD Y)| No | Yes | Yes | No |
+----------------------+------------+---------------+------------+
Figure 5: Keys per Entity
Appendix B. PERC Packet Format
Figure 6 presents a complete picture of what a PERC packet looks like
when transmitted over the wire.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A |V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
A | contributing source (CSRC) identifiers |
A | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A | RTP extension (OPTIONAL) |
A | (including the OHB) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C : :
C : Ciphertext Payload :
C : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R : :
R : EKT Field :
R : :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C = Ciphertext (encrypted and authenticated)
A = Associated Data (authenticated only)
R = neither encrypted nor authenticated, added
after Authenticated Encryption completed
Figure 6: PERC Packet Format
Authors' Addresses
Paul E. Jones
Cisco
7025 Kit Creek Rd.
Research Triangle Park, North Carolina 27709
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
David Benham
Independent
Email: dabenham@gmail.com
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Christian Groves
Independent
Melbourne
Australia
Email: Christian.Groves@nteczone.com
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